ARM: prima2: build reset code standalone

The prima2 platform code currently depends on the rstc
implementation and that in turn depends on the reset
controller framework. This removes the platform dependency
by letting the driver access arm_pm_restart directly
to turn the driver into a standalone entity, and also
removes the dependency on the reset controller framework
by using "if (IS_ENABLED(CONFIG_RESET_CONTROLLER))". This
will cause all code that is used for the reset controller
to be dropped by the compiler if the framework is disabled.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>